The present invention relates to a process for preparing diaryl carbonates by reaction of an aromatic hydroxy compound (e.g. phenol) with carbon monoxide and oxygen at elevated temperature in the presence of a base, a quaternary salt, a catalyst and a cocatalyst, which is characterized in that it is carried out in the presence of a heterogeneous promoter.
It is known that organic carbonates can be prepared by oxidative reaction of aromatic hydroxy compounds with carbon monoxide in the presence of a platinum metal catalyst (German Offenlegungsschrift 28 15 512). The platinum metal preferably used is palladium. In addition, a cocatalyst (e.g. manganese or cobalt salts), a base, a quaternary salt, various quinones or hydroquinones and desiccants can be used. The reaction can be carried out in a solvent, preferably methylene chloride.
Only small space-time yields can be achieved using these processes, so that long residence times are required. A rapid fall in the activity of the catalyst occurs during the reaction, and finally the catalyst activity is completely exhausted.
The use of various copper salts as cocatalyst proposed in EP-503 581 does not lead to higher space-time yields. Here too, the activity of the catalyst falls steadily and quickly until the catalyst is completely inactive.
The use of considerable amounts of various quinones or hydroquinones as electron transfer catalyst, as proposed in various applications (EP 350 697, EP 350 700, EP 450 442, EP 503 581, EP 550 743, etc.), improves neither the space-time yield nor does it solve the problem of deactivation of the catalyst. On the contrary, the selectivity and thus the economics of the process are considerably reduced by the formation of by-products.
The applications EP 583 935, EP 583 937, EP 583 938 relate to a cobalt catalyst containing a pentadentate ligand, specifically the cobalt di(salicylyl)-3,3'-diamino-N-methyldipropylamine (CoSMDP), which is used in combination with terpyridine. The synthesis of this complex has many stages and is complicated. Under the reaction conditions specified, these compounds are not stable so that the losses caused by decomposition have to be replaced at high cost, which makes it impossible to carry out the process economically. Even the use of such highly active and difficult-to-prepare cocatalysts cannot prevent the activity of the catalyst being reduced to half after only 2 hours.
It was therefore an object of the invention to find a catalyst system having high activity and high operating life which allows the preparation of aromatic carbonates under economical and industrially realizable conditions.